Fine particles (including nanoparticles) have a wide range of uses, with increasing world population and decreasing fossil fuel resources, renewable energy is becoming ever more in demand, and so the utility of fine particles in renewable energy applications is being considered.
One application where fine particles may be of particular use is as a component of silicon-based solar cells, such cells are the focus of much interest as a means of providing renewable energy. The basic structure of silicon solar cells includes an emitter (a positive doped area) and a base (a negative doped area) and is well known in the art. A photo effect generates charge carriers which are separated by a p-n junction, and as they cannot cross the junction, the charge carriers are conducted externally via metal contacts on both sides.
The emitter doping in standard solar cells is always a compromise as these cells have homogenous emitters. A high n-doping (for instance phosphorous) is used in the emitter layer to minimize the resistivity between semiconductor and metal contacts. However, recombination losses increase with rising n-dopant concentration, and this has an adverse impact on power generation.
One method of increasing solar cell efficiency is to use selective emitter solar cells. Here, the n-dopant concentration is selectively controlled so that only the area nearest to the contact is highly doped, and therefore has a low emitter resistivity. The remainder of the surface has a lower phosphorous doping concentration, so that as large a part of the cell as possible can be used for electricity generation—the lower dopant concentration reducing recombination losses and improving the cell's blue response. This combines an improved solar cell performance with very good ohmic contact.
The metal contact layer can use a silver paste to form the electrode system, but the cost is an issue to acceptance of this. Further the silver is often a porous paste having high contact resistance to the underlying silicon, and the adhesion to the silicon layer can be relatively poor (US2011/0318872 and Global Solar Technology, No. 3.8, page 6-10). The contact layer is usually formed using a screen printing process, but this has limitations on producing thin electrode tracks, thus causing shadowing effects that effectively prevent sunlight from reaching parts of the cell, thus reducing the efficiency.
The feasibility of replacing the silver with a low cost material has been investigated. For instance, copper has been used as a cheaper alternative to silver; however, when deposited directly onto the silicon substrate there are problems with diffusion of the copper into the silicon, which impairs the electronic properties of the cell.
Nickel has also attracted interest. A nickel seed layer can be deposited directly onto the silicon substrate to form a low resistivity nickel silicide ohmic contact. This layer can then be thickened with silver or copper plating to improve conductivity. The nickel silicide layer acts as a barrier layer between the silicon and copper preventing diffusion of the copper into the silicon. The nickel can be deposited by standard electroless plating methods (for example, as described in “Formation of a low ohmic contact nickel silicide layer on textured silicon wafers using electroless nickel plating”, A. Nguyen, et al.).
However, this deposition of the nickel layer directly onto the silicon causes consumption of the silicon substrate, which is often undesirable. Since the underlying substrate is consumed, any doping of the silicon layer, e.g., with phosphorous, will be effected. Further since silicon is the largest cost in conventional solar cell manufacture, if it is not consumed in the formation of the contacts, the amount needed is reduced, providing for significant cost savings. Thus not consuming the silicon substrate is advantageous.
There is therefore a need to improve the contact metallization of the emitter layer. The standard process basically involves three steps—i) opening contacts holes through an anti-reflective layer, ii) forming electrical contact to the silicon with good adhesion, and iii) creating a conducting path away from the contact.
The anti-reflective layer generally covers the entire cell apart from the electrical contact areas. These contact areas form a ‘shadow’ pattern on the front side of the cell and so are desirably as fine and shallow as possible. For the greatest efficiency of the cell the smaller the contact area the better. Traditional screen printing techniques are such that the shadow pattern effects the efficiency of the cell, and it would be useful to reduce the size of the contacts, hence improving the overall efficiency of the cell. The invention is intended to overcome or ameliorate at least some aspects of this problem.